The present disclosure relates to an image reading device, an image forming apparatus, and an optical unit. In particular, the present disclosure relates to the optical unit that is movable in sub scanning directions, the image reading device that includes the optical unit, and the image forming apparatus that includes the image reading device.
Image reading devices are installed in, for example, multifunction peripherals that use an electrophotographic process. A type of image reading device includes an Automatic Document Feeder (ADF) configured to sequentially forward sheets of an original document to a document table (a glass plate) so as to read images from the sheets and to discharge the sheets from the document table after the image reading process has finished.
An image reading device that includes the ADF described above is capable of reading images by implementing primarily two methods (a sheet-through method and a fixed original document method). According to the sheet-through method, an original document conveyance device automatically conveys an original document while an original document presser is kept closed, so that a scanner unit provided in the image reading device reads the images from the original document. According to the sheet-through method, the operation of reading the images from the original document is performed while the scanner unit provided in the image reading device is held in a predetermined image reading position, without being moved. According to the fixed original document method, every time the scanner unit provided in the image reading device has read an image from a sheet of original document by being moved in a sub scanning direction, another sheet of original document is placed on the document table to replace the already-read sheet, by opening and closing the original document presser.
Examples of image reading methods that can be used by a scanner unit include: a Charge-Coupled Device (CCD) method employing a charge-coupled device (a reading sensor); and a Contact Image Sensor (CIS) method employing a photoelectric conversion device (a reading sensor) called a Complementary Metal-Oxide Semiconductor (CMOS) sensor.
When a scanner unit implementing the CCD method is used, because the depth of field is deep, it is easy to bring even an original document that is not flat into focus. Thus, even if the original document (e.g., an original document in the form of a book) is not in close contact with the document table, it is possible to read images from the original document in a uniform manner. Further, scanner units implementing the CCD method have an advantage of being able to read (scan) images at a high speed. However, scanner units implementing the CCD method are disadvantageous in terms of the cost, because the structures thereof tend to be complicated and large-scaled.
In contrast, scanner units implementing the CIS method do not require any minors. Thus, it is easy to design scanner units implementing the CIS method to be thin. Scanner units implementing the CIS method are advantageous in terms of the cost, because the structures thereof are simple. However, the depth of field of CMOS sensors is shallower than that of CCD sensors. Thus, it is more difficult for scanner units implementing the CIS method to uniformly read images from an original document that is not flat. It is considered that scanner units implementing the CIS method require an adjustment with a high level of precision for the distance thereof to an original document.
Next, two image reading devices that are able to read images by implementing the two methods (the sheet-through method and the fixed original document method) will be explained.
A first image reading device includes: a document table glass plate on which an original document is loaded; an optical unit including a scanner unit, a guide shaft, and a timing belt. The scanner unit is positioned underneath the document table glass plate and is configured to read an image from an original document placed on the document table glass plate by scanning the original document in a sub scanning direction. The guide shaft is configured to support the optical unit in such a manner that the optical unit is slidable in sub scanning directions. A bearing configured to slide with respect to the guide shaft is provided in a lower part of the optical unit. The timing belt is configured to move the optical unit in the sub scanning directions.
A second image reading device includes: an optical unit including a scanner unit (a contact image sensor unit); a rail part; and a driving mechanism. The rail part is configured to support the optical unit in such a manner that the optical unit is slidable in sub scanning directions. A bearing configured to slide with respect to the rail part is provided in a lower part of the optical unit. The driving mechanism is configured to move the optical unit in the sub scanning directions.
As shown in FIG. 14, each of the first and the second image reading devices employs a shaft 101 having a circular or oval cross section, as a rail part. A bearing 102 (a sliding part) is provided in a lower part of the optical unit. The bearing 102 has a recess 102a having a semi-circular or semi-oval cross section. As another example, as shown in FIG. 15, a U-shaped bearing 103 (a sliding part) that is in contact with the shaft 101 having a circular cross section by three planes is also known.
In each of the first and the second image reading devices, the scanner unit implementing the CIS method is kept in close contact with the bottom face of the document table (e.g., the glass plate). More specifically, one or more springs are provided to bias the scanner unit upward. In addition, a sliding member configured to slide while being in contact with the document table is provided on the top face of the scanner unit. As a result of these arrangements, the distance between the scanner unit and an original document is maintained to be constant.